The film shows Joan Rivers' constant need to preserve her celebrity and her lifestyle.

Watching Joan Rivers relentlessly am bushing celebrities on the red carpet or hawking cheap jewelry on QVC, it’s easy to overlook that this plastic surgery victim is a human being, or a groundbreaking comedian.

Both these sides are exposed in Ricki Stern and Annie Sundberg’s “Joan Rivers: A Piece of Work,” a documentary that exerts a car-wreck fascination as it follows the icon through her 75th year (she’s now 77) while looking back over her tumult-filled life and career.

The film’s through-line is Rivers’ driving workaholism, which feeds both an undying need to maintain her celebrity and her lifestyle — as well as those of her staff and family. Showing off a fabulous Manhattan apartment “where Marie Antoinette would live if she had the money,” it’s small wonder the thing Rivers admits fearing most is an empty appointment book.

Even if the gigs she begs her manager to book — like one we see in Wisconsin — turn out to be far, far from glamorous.

And for all she can dish it out — with a colorful stream of profanity and Helen Keller jokes — Rivers turns out to have much thinner skin than her reputation suggests.

When an autobiographical play is poorly received in London, she cancels plans for a New York run to avoid a critical roasting here.

Her multipronged quest to stay in the game hits pay dirt when she comes up a winner on “Celebrity Apprentice” — outlasting, among others, her own daughter, Melissa.

Rivers reminds us that it’s her first major appearance in decades on NBC, where she was the first (and only) permanent guest host on “The Tonight Show.”

That ended badly, on many levels, when she accepted an offer to host a spectacularly unsuccessful late-night talk show on Fox.

Johnny Carson, her mentor, never spoke to Rivers again — and her husband, Edgar, the show’s producer, killed himself after the show was canceled.

Rivers tried to turn lemons into lemonade by starring as herself, along with Melissa, in a TV movie about her husband’s death that even she now admits was off-the-charts bizarre.

There are some rare clips from Rivers’ early ’60s TV appearances with Ed Sullivan and Jack Paar, in which she pushes the envelope of just what was allowed on the tube, with references to abortion and the casting couch.

The documentary oddly omits any mention of the Rivers-directed feature comedy “Rabbit Test,” and we don’t hear from the comedian she’s most obviously influenced, Sarah Silverman (but we do get Kathy Griffin, who played her daughter on a sitcom).

“Joan Rivers: A Piece of Work” compellingly presents a strong woman with no shortage of anger, resentment and regrets — but not an ounce of self-pity.